fetchAsync ES6 ES5

// http://es6-features.org/

// ECMAScript 6
function fetchAsync (url, timeout, onData, onError) {
    …
}
let fetchPromised = (url, timeout) => {
    return new Promise((resolve, reject) => {
        fetchAsync(url, timeout, resolve, reject)
    })
}
Promise.all([
    fetchPromised("http://backend/foo.txt", 500),
    fetchPromised("http://backend/bar.txt", 500),
    fetchPromised("http://backend/baz.txt", 500)
]).then((data) => {
    let [ foo, bar, baz ] = data
    console.log(`success: foo=${foo} bar=${bar} baz=${baz}`)
}, (err) => {
    console.log(`error: ${err}`)
})

// ECMAScript 5
function fetchAsync (url, timeout, onData, onError) {

}
function fetchAll (request, onData, onError) {
    var result = [], results = 0;
    for (var i = 0; i < request.length; i++) {
        result[i] = null;
        (function (i) {
            fetchAsync(request[i].url, request[i].timeout, function (data) {
                result[i] = data;
                if (++results === request.length)
                    onData(result);
            }, onError);
        })(i);
    }
}
fetchAll([
    { url: "http://backend/foo.txt", timeout: 500 },
    { url: "http://backend/bar.txt", timeout: 500 },
    { url: "http://backend/baz.txt", timeout: 500 }
], function (data) {
    var foo = data[0], bar = data[1], baz = data[2];
    console.log("success: foo=" + foo + " bar=" + bar + " baz=" + baz);
}, function (err) {
    console.log("error: " + err);
});